Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 200456

Area Forecast Discussion
National Weather Service Riverton WY
1056 PM MDT Mon Mar 19 2018

.SHORT TERM...This afternoon through Thursday
Issued at 1243 PM MDT Mon Mar 19 2018

Isolated to scattered showers are expected this afternoon into the
evening across northern portions, as a weak trough moves over
western/southern MT. Some of this will also be driven by diabatic
heating, as the CU field develops over the area this afternoon.
Snow will mainly be confined to the higher elevations, before
temperatures fall tonight in the lower elevations. There will
be a chance to slight chance of snow showers to continue over the
western and northern mountains Tuesday, as weak upslope flow
continues. Another weak shortwave will also help produce these
showers over the west.

Otherwise, the period will be marked by a warming trend as a ridge
of high pressure builds over the area. Precipitation chances will
remain confined to the west/northwest, as moist upslope flow
continues. Any rain Tuesday afternoon is expected to change back
over to snow overnight Tuesday, before changing back to rain by
midday Wednesday. This is expected to lead to rain on snow to
those low elevation locations in the Jackson/Star Valleys and
Upper Green River Basin that still have snow on the ground. This
will continue Thursday and increase any low land flooding risk, as
precipitation amounts will be greater compared to Wednesday. This
will also affect the mountainous areas, as snow levels will range
between 7500 to 8500 feet on Wednesday, possibly increasing to
9000 feet on Thursday (especially for the Salt/Wyoming Ranges).

.LONG TERM...Thursday night through Monday night
Issued at 1243 PM MDT Mon Mar 19 2018

Beginning of forecast: Trough/ridge/trough pattern in place across
the CONUS with passing ridge axis just to the east of the forecast
area roughly along the eastern border of Wyoming. Upper level flow
has transitioned to southwest with the nose of a broad strong jet
and associated embedded shortwave moving into/over western WY along
with what looks to be decent Spring moisture from the EPAC and then
across the southwestern CONUS. In reality, will have to see just how
much actually makes it to the CWA after bumping into/over the Sierra
Mountain range. Regardless, it will be raining/snowing west of the
Divide in Wyoming.

Overall, MR models are in decent agreement through the day Saturday
before more significant differences begin to show. Regardless of
differences, however, the region does stay under the influence of
the western CONUS trough through the end of the forecast period with
the best chance for precipitation (overall) occurring during this
period of the forecast. (Particularly between Thursday night and
Sunday morning). Sunday and beyond, the main difference between the
EC and GFS occur within the trof as the EC evolves a closed low
while the GFS remains open. Both are progressive, however, and the
model solutions with respect to precipitation areas and amounts
actually are quite close (the EC is somewhat more expansive with
precipitation due to the closed system`s area of influence). Sunday
night through the end of the forecast, models differ somewhat in
their respective exit strategies, but all stick with the idea of
finally pushing the main upper trough east/northeast and out of the

Possible impacts during the Thursday night through Saturday period
will include: 1. Potential periods of moderate to occasionally heavy
rain/snowfall (convective) with possible low end flooding concerns
for portions of Lincoln County due to increased/added snowmelt
leading up to/through these periods. 2. Advisory to possible Warning
level snowfall across the western mountains. 3. Brisk, potentially
high, winds across portions of the Wind Corridor Friday and/or
Saturday. 4. A few isolated Thunderstorms possible Friday...mainly
across the west/southwest portion of Wyoming.


.AVIATION...For the 06Z TAFS through 06Z Wednesday
Issued at 1056 PM MDT Mon Mar 19 2018

Isolated to scattered snow showers over the western mountains
through Tuesday night. Areas of MVFR to IFR conditions will occur
vcnty the mountains. KJAC terminal will have VCSH through 02Z Wed.
Otherwise, VFR conditions will occur at the terminal sites.


Issued AT 1243 PM MDT Mon Mar 19 2018

Isolated to scattered snow showers will redevelop over the
northwestern and northern mountains this afternoon into this
evening. Precipitation will be in the form of rain (along the Cody
Foothills) or mixed with snow (along the foothills of the
Absarokas and Bighorns). Elsewhere, conditions will mainly be dry
across the area, with seasonal temperatures and gusty winds across
the south. A warming trend will begin Tuesday and continue
through the week. Gusty winds will also return across wind prone
locations (mainly during the afternoon) from Tuesday through
Thursday. At the same time, mixing heights will also continue to
increase through the week. Smoke dispersal will also be fair to





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